Wireless communications is becoming increasingly important, with wireless systems finding their way into every growing numbers of applications. At the same time, increasing demands are being made on wireless communications systems. Modern telecommunications systems are often expected to handle a wide variety of data types. For example, communications systems may be asked to carry voice, computer data, image files, video, and similar types of data. Such communications systems are sometimes referred to as multimedia communications systems. In some cases, communications systems that were initially designed to carry only one type of data are being asked to carry additional types of data. For example, cellular voice systems have extended to include data transmission (e.g., text messaging and email) and are now being further extended to provide internet access and video transmission.
The differing data types in a multimedia communication system often have different requirements as to how they are handled by the system. For example, voice data is typically quite sensitive to latency, and end-to-end transport delays of less than about 150 milliseconds are often desirable. Voice data is also relatively tolerant of errors, and thus occasional transmission errors are typically acceptable. In contrast, computer data can have widely varying requirements, depending on the application. Some data is relatively time insensitive, and delays of many seconds are acceptable, while other data is worthless if not received within fractions of a second. Computer data must often be transferred error free. In additional to latency and error rate, other performance characteristics are often important in a communication system.
The term “quality of service” (QoS) refers to system performance characteristics that are desired. For example, QoS requirements can be defined in terms of performance characteristics such as data rate, transport delay, transport delay variability (jitter), data (packet) loss probability, data bit error probability, undetected error rate, round-trip delay, or like characteristics. While the foregoing parameters are primarily applicable to digital data, QoS characteristics can be defined in terms more traditionally associated with analog signals, such as signal-to-noise ratio, signal-to-interference ratio, transport channel frequency response, level of echo, and like characteristics. The foregoing list is not intended to be exhaustive, and other performance characteristics not mentioned above may be included within the definition of a particular QoS.
Wireless communications systems can prove particularly challenging when used for multimedia communications. The wireless communication channel is inherently unreliable and changing, and can experience errors, loss of data, interference, as well as significantly differing conditions on paths to different sources and destinations. Considerable research and development has been expended in addressing these challenges. While optimal solutions are known to some theoretical communications problems, most systems designs have generally represented a number of sub-optimal compromises. In particular, most wireless system designs have been optimized for one particular type of data, and provide sub-optimal performance when used for other types of data. Most wireless systems deployed to date use a fixed physical layer format, which is optimized for one data type, and other data types are made to fit within the system characteristics. While such designs have achieved significant market success, there is increasing desire for wireless communications systems to provide good performance for a mixture of data types.